1. Field of the Invention
The present invention relates to a handover communication technology for a mobile terminal in a mobile communication system.
2. Description of the Related Art
A communication system, which is generically termed an LTE (Long Term Evolution), exists as a fast data communication system for mobile equipment, of which standardization is now progressed by 3GPP (3rd Generation Partnership Project). The LTE is ranked as a stage for further evolving a W-CDMA (Wideband-Code Division Multiple Access) system etc called 3G and an HSDPA (High Speed Downlink Packet Access) system etc called 3.5G and for attaining a smooth shift scheme to the next generation (4G).
FIG. 25 is a diagram showing an example of a logical architecture of an LTE mobile communication system. As illustrated in FIG. 25, the LTE mobile communication system at the present stage (which will hereinafter be referred to as a conventional LTE mobile communication system) is built up by an evolved Node B (which will hereinafter be abbreviated to eNB) 511, an MME (Mobility Management Entity) 512, an SAE-GW (System Architecture Evolution GateWay) 513, an HSS (Home Subscriber Server) 514, a PDN-SAE-GW (Packet Data Network SAE GW) 515, a user terminal (which will hereinafter be referred to as UE (User Equipment)) 516, etc.
Roles of respective nodes configuring this type of LTE mobile communication system are defined as follows. The eNB 511 has a role as a wireless base station. The SAE-GW 513 has a role as a gateway that mainly manages a user plane (which will hereinafter be abbreviated to the U-plane) for controlling transmission and reception of user data between users. The MME 512 has a role as a gateway that mainly controls a user call for controlling the U-plane, manages a control plane (which will hereinafter be abbreviated to a C-plane) for controlling a connection etc, and performs movement management control of the UE 516. The HSS 514 has a role as a server that retains subscriber data, and the PDN-SAE-GW 515 has a role as a gateway that manages the SAE-GW 513 and connects the self-system to an external network such as an IMS (IP Multimedia Subsystem) 510. Thus, the LTE mobile communication system distributes a function of a conventional RNC (Radio Network Controller) among the eNB 511, the MME 512, the SAE-GW 513, etc and omits the RNC node itself.
As for a topology, an interface connecting the respective eNBs 511 to each other is called an X2 interface (X2 in FIG. 25), and an interface connecting between the eNB 511, the MME 512 and the SAE-GW 513 is called an S1 interface.
In this type of LTE mobile communication system also, the handover process accompanying a movement of the UE 516 is important. Namely, when the UE 516 moves from a communication source base station (which will hereinafter be termed S-eNB (Source-eNB)) to a communication destination base station (which will hereinafter be termed T-eNB (Target-eNB)), it is required that all the user data are forwarded without setting in a communication-disconnected status and causing any loss. An examination of a technique for realizing this type of no-loss handover in the LTE mobile communication system is now underway. In the handovers, a handover, in which a management unit of the MME 512 and the SAE-GW 513 changes before and after the UE 516 has moved, and the X2 interface between the S-eNB and the T-eNB can not be utilized, will hereinafter be expressed as an S1 base handover.
An outline of the S1 base handover process in the conventional LTE mobile communication system will be explained with reference to FIGS. 26 and 27. FIGS. 26 and 27 are diagrams each showing the outline of the S1 base handover process in the conventional LTE mobile communication system.
The S-eNB 521 determines the handover for the UE 529 on the basis of reception quality information etc given from the UE 529 (S1). When the S-eNB 521 determined the handover, an S-MME 523, a T-MME 524, a T-eNB 522, a T-SAE-GW 526 transmit and receive a C-plane control message shown in FIG. 26, thereby notifying a PDN-SAE-GW 528 of the handover for the UE 529 (S2), (S3), (S4a), etc.).
On the other hand, when a preparation for the handover for the UE 529 is completed on the system side by transmitting and receiving the C-plane control message, the S-eNB 521 sends a Handover Command message to the UE 529 (S9). With this operation, the UE 529 secedes from a precedent cell managed by the S-eNB 521 and starts a process of synchronizing with a present cell managed by the T-eNB 522.
The UE 529, upon taking the synchronism with the present cell, sends a Handover Confirm message to the T-eNB 522 in order to notify of an approval of the handover (S11). The T-eNB 522 receives this message, thereby detecting the approval of the handover for the UE 529. Thereafter, the T-eNB 522 sends a Relocation Complete message to the T-MME 524 (S12). Subsequently, the T-MME 524 transmits an Update Context Request (SAE-GW) message to the T-SAE-GW 526 (S14).
Finally, the PDN-SAE-GW 528 detects the approval of the handover for the UE 529 by receiving the Update Context Request (PDN-SAE-GW) message from the T-SAE-GW 526 (S15a).
Note that the following documents are disclosed as the documents of the related arts related to the invention of the present application. The first document is a “Japanese Patent Laid-Open Publication No. 2006-80690”. The second document is a “Japanese Patent Laid-Open Publication No. 2007-104344”. The third document is a “Ericsson, “Inter eNodeB handover with CN node relocation (Discussion/Approval) (Agenda Item 7.8)”, 3rd Generation Partnership Project, 3GPP TSG-RAN WG3 Meeting #55bis (R3-070623), Mar. 27-30, 2007”. The fourth document is a “Technical Specification Group Services and System Aspects, “GPRS enhancements for E-UTRAN access (Release 8)”, 3rd Generation Partnership Project, 3GPP TS 23.401 V1.3.0, October 2007, p. 73-76”. The fifth document is a “Technical Specification Group Services and System Aspects, “Architecture Enhancements for non-3GPP accesses (Release 8)”, 3rd Generation Partnership Project, 3GPP TS 23.402 V1.4.0, October 2007”. The sixth document is a “Technical Specification Group Radio Access Network, “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN) (Stage 2-Release 8)”, 3rd Generation Partnership Project, 3GPP TS 36.300 V8.2.0, September 2007”.
In the conventional LTE mobile communication system described above, when taking the handover in the status where the management unit of the MME 512 and the SAE-GW 513 remains unchanged, if the S-eNB and the T-eNB mutually transmit and receive the uplink data or the downlink data via the X2 interface, the no-loss handover can be actualized. When taking the S1 base handover, however, it is not an easy to realize the no-loss handover.
The S1 base handover in the conventional LTE mobile communication system causes a time lag till the PDN-SAE-GW 528 detects an approval of a handover since the UE 529 actually has approved a connection handover from the S-eNB 521 to the T-eNB 522. To be specific, the UE 529 has already approved the connection handover just when the Handover Confirm message is sent to the T-eNB 522 (S11). The PDN-SAE-GW 528, however, finally comes to know the connection handover of the UE 529 from the Update Context Request (PDN-SAE-GW) message (S15a) through (S12) and (S14) after the T-eNB 522 has received the Handover Confirm message.
Accordingly, when the data is downlinked to the UE 529 from the PDN-SAE-GW 528, the UE 529 takes the handover, in which case the connection target of the UE 529 has already been switched over to the T-eNB 522, and nevertheless it follows that the PDN-SAE-GW 528 continues to transmit the downlink data for the UE 529 to the S-eNB 521 during the time lag.
At this time, if not the S1 base handover, the X2 interface can be utilized, and hence the S-eNB 521 may forward the downlink data to the T-eNB 522 via the X2 interface (S10). The S1 base handover illustrated in FIGS. 26 and 27 is, however, disabled from using the X2 interface because the management unit of the MME and the SAE-GW changes before and after the UE has moved (from the S-MME 523 to the T-MME 524, and from the S-SAE-GW 525 to the T-SAE-GW 526) ((S10) in FIG. 27).
Such being the case, what is considered is a technique of selecting a route (S10′) such as S-eNB 521→S-SAE-GW 525→PDN-SAE-GW→T-SAE-GW→T-eNB 522 in order to transmit, to the T-eNB 522, the data that has already been sent to the S-eNB 521 during the time lag. This technique, however, causes a large time lag due to a multiplicity of via-nodes, and further comes to wastefully consume communication bandwidths between the individual nodes. Note that only a signaling route on the C-plane is established between the S-MME 523 and the T-MME 524, and therefore this C-plane route can not be utilized for forwarding the user data.
Further, if the UE 529 takes the S1 handover when the data is uplinked to the PDN-SAE-GW 528 from the UE 529, the PDN-SAE-GW 528 can not take sequence matching of the uplink data, and hence it is required that, after the T-eNB 522 has taken the sequence matching, the data are uplinked in a due sequence to the PDN-SAE-GW 528 via the T-SAE-GW 526. If not the S1 base handover, the X2 interface can be utilized, so that the S-eNB 521 may forward the uplink data to the T-eNB 522 via the X2 interface (S10). The S1 base handover can not, however, utilize this X2 interface ((S10) in FIG. 27).
Accordingly, a thinkable technique about the S1 base handover is a technique of transmitting the uplink data received by the S-eNB 521 to the T-eNB 522 via the same route (S10′) as when downlinked. This technique, however, causes the large time lag due to the multiplicity of via nodes as when downlinked, and further comes to wastefully consume the bandwidths between the respective nodes.